New grain growth experiments in water ice

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Abstract:

Grain growth exponents are constrained independently of the thermally activated kinetics of growth. Two homogeneous samples with dissimilar grain sizes were subjected to a single heating event. The exponential n is proportional to the ratio of their change in grain size (D) (see thesis for equation).

Heat treatment experiments were performed in superfine (< 25um) ice samples of variable porosity. A new type of superfine ice was synthesised. Spherical ice particles (2-30um) were generated by using an inkjet piezoelectric printer to spray micro-droplets of deionised water into liquid nitrogen. The micro-powder was compressed uniaxially to form rock ice. Ice sphere characteristics give some insights into very low temperature phase transformations in ice.

A cooling system based on the thermoelectric properties of the peltier chip was developed to apply the heat treatments (min T = -22C). The grain size data was collected qualitatively, using cryo-EBSD. Both sensible (1/n > 1) and nonsensical (1/n < 1) exponents were found. The sensible data gave inverse exponents between 2 and 40, which increased with porosity. Growth rates in my synthetic porous ices are less than those for fully dense, pure synthetic ice and greater than the rates observed in terrestrial ice and firn.

The nonsensical exponentials were found in experiments where the two samples grew according to different kinetics. This violated the underlying assumption of the mathematical reasoning of the method.

The exponents found compare favourably with those found with the same data using the traditional method to determine n. The exponents vary significantly with the different averaging tool used to determine grain size. The largest source of error in the proposed methodology stems from the non-similarity of the paired samples. There is a significant portion of error from measurement errors in D. This makes for imprecise exponents. The imprecision in n values found with this method is also present in n constrained using the traditional method. Care is required when quoting growth rates in terms of n.